Cardiac pacemakers stimulate a patient's heart by applying current pulses to cardiac tissue via two electrodes, a cathode and an anode. Standard pacing leads exist in either of two configurations, unipolar leads or bipolar leads, depending on the arrangement of the electrodes of a particular lead. A unipolar pacing lead contains a single electrode, normally the cathode, which extends pervenously distal from the pacemaker in an insulating enclosure until it is adjacent to the tip of the lead where the insulation is terminated to provide for electrical contact of the cathode with the heart. The anode provides a return path for the pacing electrical circuit. For a unipolar lead, the anode is the pacemaker case. A bipolar lead contains two electrodes within an insulating sheath, an anode which extends distal from the pacemaker to a position adjacent to but spaced from the electrode tip, and a cathode which also extends distal from the pacemaker but terminates a short distance distal of the anode, at the lead tip. The insulation-exposed anode commonly takes the form of a ring. The cathode and anode of a bipolar lead are separated by an insulating barrier. In present-day pacemakers, circuits for pacing and sensing, which determine tip, ring and case electrode connections, are provided. Thus the pacemakers can be programmed via telemetry for either bipolar or unipolar operation with respect to either sensing or pacing operations.
One problem that persists in present-day pacemakers is that of guaranteeing the pacemaker will not be programmed into a bipolar mode when it is connected to a unipolar lead. A similar problem arises in some rate-responsive pacemakers requiring implantation of a bipolar lead in the heart to sense a physiological parameter which may then be used to determine a metabolic demand pacing rate (see U.S. Pat. No. 4,901,725, entitled "Minute Volume Rate-Responsive Pacemaker", issued to T.A. Nappholz et al. on Feb, 20, 1990). Upon occasion a unipolar lead has been inadvertently implanted with such a rate-responsive pacemaker, and the device has had to be explanted because it failed to rate-respond. A lead polarity test at the time of implant, together with an appropriate warning of the implantation of a lead which is inconsistent with device requirements, could prevent an explantation procedure. Attempts have been made to solve these problems.
In U.S. Pat. No. 4,532,931, entitled "Pacemaker with Adaptive Sensing Means for use with Unipolar or Bipolar Leads", issued on Aug. 6, 1985, P.A. Mills discloses a sensing circuit for a cardiac stimulator capable of automatically adapting to use with either bipolar or unipolar leads without the necessity for telemetric programming of a polarity parameter contained within the implanted pacemaker. The circuit senses artifacts and intrinsic cardiac signals, such as R-waves, on an implanted lead to determine lead polarity. If a unipolar lead is connected to a terminal receptacle of the pacer at the time of implant, the pacer will sense electrical signals between a distal tip electrode and the metal body of the pacemaker. If a bipolar lead is attached to the terminal receptacle at the time of implant, the pacer will sense electrical signals between a tip electrode and a ring electrode spaced a short distance proximal to the distal tip electrode. The pacemaker determines lead polarity through analysis of impedance ratios and voltage division at the input terminals of the pacemakers sensing amplifier. It measures intrinsic activity or artifact signals for both the unipolar and bipolar configurations to determine whether a bipolar lead is present.
One problem with the Mills pacemaker is that it requires the sensing of cardiac electrical signals and artifacts for its analysis. Therefore, the pacemaker requires a lead which is appropriately implanted within the heart and capable of sensing electrical signals within a reasonable range of amplitudes for the polarity test feature to operate in a reliable manner. Furthermore, the pacemaker requires a somewhat appropriate setting of the sensitivity of the amplifiers within the sensing circuit for viable operation.
U.S. Pat. No. 4,964,407, entitled "Method and Apparatus for Assuring Pacer Programming is Compatible with the Lead", issued on Oct. 23, 1990 to R.G. Baker et al. addresses some of the problems of the Mills invention. Baker et al. teaches a pacemaker which employs an internal microprocessor which generates a series of test signals to determine whether a bipolar or unipolar lead is attached to the lead connector of the pacemaker. The pacemaker restricts its telemetric programming to a unipolar mode unless it has detected the presence of a bipolar lead. The pacemaker performs this detection by generating a series of low frequency noise pulses via a high impedance circuit, applying these pulses to lead contacts and sensing a return signal. If a return signal is detected, this indicates either that a unipolar lead is connected to the pacemaker or that the ring conductor of a bipolar lead is open and bipolar pacing must be inhibited.
A disadvantage of the Baker et al. pacemaker is that it requires additional circuitry for generating the test signals. These additional circuits add to the size and energy requirements of the pacemaker. A very important consideration in implanted cardiac pacemakers is restraint of size and energy requirements.
The present invention provides for determination of lead polarity but does not require either the sensing of intrinsic heart signals or the generation of test signals. The present invention is based upon the premise that lead polarity can be determined using a measurement of lead impedance, also called electrode impedance. A lead impedance measurement value larger than a predetermined threshold level indicates either that no lead is present, that an electrode is broken or that the programming of the polarity mode is incorrect. A cardiac stimulation device, by measuring lead impedance during a bipolar pace, can determine if a bipolar lead is present. If the bipolar lead impedance measurement yields a value which is below the threshold impedance, a bipolar lead is connected to the stimulator. Otherwise, the stimulator may perform a unipolar lead impedance measurement to determine whether a viable unipolar lead is attached. Failure of the unipolar lead impedance test indicates either that no lead is attached to the stimulator or that the attached lead is broken.
The measurement of lead impedance is known in the art of cardiac pacemakers. For example, U.S. Pat. No. 4,448,196, entitled "Delta Modulator for Measuring Voltage Levels in a Heart Pacer", which issued on May 15, 1984 to D.K. Money et al., teaches a measurement circuit within a pacemaker which monitors lead impedance of a pacing lead by sensing the decrease in amplitude between the leading and trailing edges of a pacing pulse generated by the pacemaker as it performs its standard pacing function. The voltage of an output capacitor, the power source for tissue stimulation pulses, is sampled both before and after a stimulation pulse. The difference in voltage level between the two samples is caused by a partial discharge of the output capacitor during delivery of the stimulation pulse. This difference voltage value is used to determine the instantaneous lead impedance.
The lead polarity determination system of the present invention requires more than the usage of a lead impedance measurement to determine lead polarity to provide a viable stimulation device. The stimulator must continuously provide pacing support to a patient. If a unipolar lead is in place, the patient is not supported when a bipolar mode pacing pulse is delivered. Therefore, it is necessary to support unipolar pacing during each cardiac cycle of the lead polarity test. Thus the system of the present invention follows each bipolar pace with a unipolar backup pacing pulse shortly subsequent to the bipolar pace. In the preferred embodiment of the invention, the unipolar pace is delivered approximately 100 ms following the bipolar pulse. Note that if a bipolar lead is present, the bipolar pace supports the patient and no unipolar pace need be delivered.
Thus, it is an object of the present invention to test the polarity of a lead attached to an implanted stimulation device using a lead impedance measurement.
It is a further object of the present invention to automatically set the polarity mode of an implanted stimulation device to match a lead attached thereto on the basis of the results of a lead impedance measurement.
It is another object of the present invention to test the polarity of a lead attached to an implanted stimulation device using a lead impedance measurement which is functional via the sensing of a decrease in amplitude between the leading and trailing edges of a pacing pulse generated by the stimulator as it performs its standard pacing function.
It is an additional object of the present invention to test the polarity of a lead attached to an implanted stimulation device using a lead impedance measurement performed during the delivery of pacing pulses in which a bipolar pace measurement is followed by a unipolar backup pacing pulse shortly subsequent to the bipolar pace to assure the safety of a patient.
Further objects and advantages of the invention will become apparent as the following description proceeds.